The present invention relates to a system for the transmission of information between at least two stations for transmitting information from free or engaged channels, each station having at least one radio transmitting and one reception circuit, powered from a power supply and means for detecting the free or engaged channels.
Such a system is known and described in the publication "COMMUTATION ET TRANSMISSION", No. 3, September, 1981, in an article entitled "Systeme de telephone rurale IRT 1500" ("IRT 1500 rural telephone system"). The article describes a numeric rural telephone system for low-density traffic in which a central and distant stations are linked by radio. In the IRT 1500 system, the distant stations are usually in remote areas, often without a mains electricity supply. They therefore often have to be solar-powered. It is therefore essential to keep the power consumption of the various stations low, and one way of doing so is to use CMOS technology in the design of the equipment.
It must nevertheless be admitted that the equipment in the stations like transmission and reception circuits are continuously powered and consume energy even when there is no information to transmit, at times of very low-density traffic, especially at night, for example.
The aim of the invention is to eliminate this drawback, and it proposes means of very substantially reducing power consumption.
Here, the information transmission system of the type described in the preamble is noteworthy in that it has means working together with the detection means to set up a process for creating a time multiplex consisting of an elementary field of duration T in which the information from engaged channels is grouped side-by-side in said elementary field and means for reducing the consumption of the transmission and reception circuits from the detection of free channels and to provide a normal power supply for the transmission and reception circuits from the detection of engaged channels.
The idea of the invention is based on the fact that, in order to make a substantial saving in the power of the various stations, the transmission and reception circuits are to operate only when there is useful information to transmit.
In addition, the multiplex field is so organised that the channels in continuous use, i.e. the "service" channels (rhythm, synchronisation, signalling, etc.) and the "engaged" channels allocated to the request are grouped, for instance, at the beginning of the multiplex field which means that, if traffic is low, there is a large number of unused contiguous channels at the end of the field. As soon as a channel becomes free, it is used as a matter of priority until a fresh allocation is requested. The signalling channels carry information showing which channels are in use and the number of the last channel used in the field. The transmission and reception circuits may thus be cut off from the last channel used. They must, however, be re-activated before the start of the new field to take account of their response time.
If, nevertheless, the response time of the transmission and reception circuits is smaller than the duration T of a multiplex field, it is still of the same order of magnitude and the resultant power saving is fairly small. Therefore, and in order to increase power savings very substantially, the influence of the response time of the transmission and reception circuits must be reduced.